Commitments and Contingencies, Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Operating Leases, Future Minimum Payments Due [Abstract]
2014	$ 61.4
2015	53.1
2016	53.1
2017	52.2
2018	51.8
Thereafter	746.8
Purchased Power Agreements (PPAs)
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations	59.0		56.0	54.4
Operating Leases, Future Minimum Payments Due [Abstract]
2014	58.3	[1],[2]
2015	49.9	[1],[2]
2016	49.9	[1],[2]
2017	49.9	[1],[2]
2018	49.9	[1],[2]
Thereafter	734.1	[1],[2]
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations	64.2		59.9	58.8
Operating Leases, Future Minimum Payments Due [Abstract]
2014	3.1
2015	3.2
2016	3.2
2017	2.3
2018	1.9
Thereafter	$ 12.7

[1] PPA operating leases contractually expire through 2033.

[2] Amounts do not include PPAs accounted for as executory contracts.